hozan23 opened a new pull request, #26886:
URL: https://github.com/apache/datafusion/pull/26886
## Which issue does this PR close?
- Closes #26885.
## Rationale for this change
`MySqlDialect` unparses every timestamp cast as `DATETIME`, which MySQL
treats as `DATETIME(0)`. Sub-second values are rounded to whole seconds.
See https://dev.mysql.com/doc/refman/8.0/en/fractional-seconds.html
## What changes are included in this PR?
`MySqlDialect::timestamp_cast_dtype` derives the precision from the time
unit: `DATETIME` for seconds, `DATETIME(3)` for milliseconds, and `DATETIME(6)`
for micro- and nanoseconds (MySQL allows at most 6).
## What is the testing strategy for this PR?
New unit test `mysql_timestamp_cast_keeps_fractional_precision` in
`datafusion/sql/src/unparser/expr.rs`.
## Are there any user-facing changes?
No API changes.
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